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Rain and runoff

The eastern Valley received some much-needed rain in May. But, as the chart below shows, the pattern of dry weather observed earlier in the year resumed in June.

Month

Observed rainfall

Normal rainfall

Percent of normal

January

4.51

4.6

98

February

2.68

4.23

63

March

2.79

4.82

58

April

2.69

4.17

65

May

4.50

4.23

106

June

2.91

4.28

70

Eastern Valley rainfall for the year to date is 76 percent of normal.

Runoff (the amount of water that reaches the river system when it rains instead of being absorbed into the ground) is currently 91 percent of normal in the eastern Valley.

Reservoir elevations

Despite the dry weather in June, TVA has been able to keep most tributary reservoirs very close to their flood guide levels by releasing only the minimum amount of water needed to meet downstream flow requirements. The exception is Cherokee Reservoir, which was still one-and-a-half feet below its flood guide elevation on July 1. TVA was able to bring Cherokee water levels up several feet in June by pulling the water needed to meet the system flow requirements from those reservoirs at higher levels and will continue to conserve water in Cherokee until it is in balance with other tributary reservoirs.

Tributary reservoir elevations¹

Reservoir

July 1, 2010Observed Elevation

July 1Flood GuideElevation2

South Holston

1728.6

1729

Watauga

1956.7

1959

Cherokee

1067.5

1069

Douglas

992.4

992.7

Fontana

1702.9

1703

Norris

1019.7

1020

Chatuge

1924.9

1925

Nottely

1775.6

1776

Hiwassee

1519.9

1520

Blue Ridge

1685.9

1686

Tims Ford

888.1

888

Normandy

874.9

875

¹Elevations above mean sea level
²Flood guide elevations show the amount of storage allocated for flood damage reduction during different times of the year. From June 1 through Labor Day, TVA's goal is to meet downstream flow requirements while keeping the reservoir elevation at the dam as close to the flood guide level as possible to support reservoir recreation. During this period, reservoir elevations fall below the flood guide only when rainfall and runoff are insufficient to meet flow requirements. The rest of the year, the primary objective is to keep the reservoir elevations at or below the flood guide to ensure there is enough space in the reservoir to store the rain and runoff from flood events.

Reservoir operations

TVA’s challenge in the coming weeks will be to keep water levels in tributary reservoirs near target levels.

That’s according to Chuck Bach, TVA General Manager for River Scheduling. “Our flow commitments increase through the summer in order to protect water quality as river temperatures increase as well as to maintain the flows needed for other river system benefits," he says. That means we have to release more water from the tributary reservoir system, which will make it hard to keep tributary reservoirs at their flood guide levels without more rain.”

But Bach says TVA will certainly try. “Our operating strategy is to conserve every drop of water in the tributary reservoir system that we can to help maintain summer pool levels. We will continue to generate hydroelectric power with the water we release to meet our flow requirements. But we will not release any extra water for power purposes.”

“Using river water to cool our nuclear and fossil plants saves a lot of money. But water used for cooling is heated in the process. When that water is returned to the river, it can cause downstream water temperatures to exceed the plant’s permit limits especially under low-flow conditions. In the summer, we try to conserve water in upstream reservoirs, so instead of providing the flow we would need to keep river temperatures where they should be, we use cooling towers or reduce generation at our nuclear- and coal-fired plants.”

Balancing tributary reservoir elevations will be another key focus in the weeks ahead, says Bach.

He adds that if TVA needs to release water from the tributary reservoir system to meet downstream flow requirements, it will follow reservoir balancing guidelines to make sure it is released equitably.

When extra water is needed to meet downstream flow requirements, TVA looks at the elevation of each reservoir is relation to its balancing guide and flood guide. Then TVA decides how much water to withdraw from specific reservoirs in order to bring reservoir elevations into balance - in other words, to keep the elevation of each reservoir similar relative to its position between the flood guide and the balancing guide.

Go to TVA’s River Management page for current information about rain, reservoir elevations, and releases.

In answer to your question about summer reservoir elevations

If TVA restricts releases from my reservoir during the summer, why does the pool level sometimes drop before Labor Day?

The fact that releases are restricted during the summer doesn’t mean there won’t be any drawdown prior to Labor Day.

Summers in the Tennessee Valley are typically hot and dry. On average, only about an inch of rain reaches the reservoir system per month during June, July, and August, and much of that water quickly evaporates. As a result, in most years, there isn’t enough water coming into the tributary reservoir system to replace the water TVA must release to meet downstream minimum flow requirements, which causes a gradual drop in tributary reservoir elevations through the summer.

Minimum flow requirements are set to ensure an adequate flow of water through the river system. They specify the minimum amount of water needed to meet a broad range of river system operating objectives, such as protecting aquatic life below tributary dams by keeping the riverbed from drying out, maintaining suitable depths for navigation on the main Tennessee River, keeping industrial and municipal intakes covered with water, meeting recreation release commitments, and providing the water needed to cool nuclear and coal-fired power plants and spin the turbines at hydro-electric plants.

Flood drills help TVA prepare for the worst

A hurricane has made landfall on the Gulf coast and is tracking generally toward the northeast. The National Weather Service predicts that it will stall out over the eastern part of the Tennessee Valley. The forecast calls for 8 to 12 inches of rain at the very least, with some locations getting close to a devastating 30 inches.

And it gets worse...

The remnants of another tropical storm brought very heavy rainfall to the eastern Valley just a few days earlier. TVA has been moving that water through the system as quickly as possible, but many reservoirs are still above target levels.

It's a sobering scenario, but not at all unrealistic. In fact, that was exactly the situation TVA faced in September 2004 after Hurricanes Frances and Ivan delivered a one-two punch to the Gulf coast. And it is exactly the kind of situation that TVA prepares for and trains for.

“Hopefully, most of our River Forecast Center staff will never have to deal with that kind of a situation,” said David Bowling, Senior Management Advisor for TVA River Scheduling. “But we conduct regular drills to make sure they are ready.”

A lot of planning goes into each drill, according to Bowling.

“We try to simulate a real event as closely as possible, including the uncertainty involved in dealing with weather events and the sense of urgency involved when people's lives and property are at stake.

“The scenarios vary. Sometimes we use a storm that was forecast to move into the Valley, but changed tracks. Other times, we transpose an actual storm to a different location. Our Forecast Center staff puts the predicted rain into our computer models, determines potential impacts, and evaluates different operating strategies with the goal of minimizing as much damage as possible.

“We also practice implementing our communication plans—for example, working with the National Weather Service in predicting flood levels and with local emergency management agencies in issuing warnings to the public.”

December through early May is the major flood season in the Tennessee Valley because runoff—the amount of water that ends up in the river system after it rains—is higher and because storms tend to be larger in the winter and early spring. But there is potential for serious flooding even in June, July, and August due mainly to the remnants of tropical storms and hurricanes.

Being well prepared for those types of events is critical, says Bowling. “We reserve less flood storage space in TVA-managed reservoirs in the summer because there's less chance of a major flood-producing storm. Plus, allowing reservoirs to fill to higher levels gives us more water in storage, which helps to ensure that we will have water available as needed to support navigation, water quality, recreation, and other operating objectives. But it also means we've got to be ready to respond quickly and correctly in the event of a major storm in order to minimize the damage. That's when the experience we gain in simulating storms can really pay off.”

The National Weather Service is predicting an 'active to extremely active' hurricane season for the Atlantic basin this year, so that payoff could come soon.

“You can't control when a hurricane or other emergency may happen,” says Bowling, “but you can make sure you're ready—and that's exactly what we're doing.”

Aeration season is in full swing

It happens every summer. As the days get longer and hotter, the temperature of the surface water rises. Since warm water is less dense than cold water, it literally floats on top of the cooler water. This density difference inhibits mixing, resulting in thermal stratification—the separation of water into horizontal layers due to temperature differences.

So what's the problem? If the layers don't mix, the bottom water becomes trapped. The oxygen in the deeper water isn't replenished by contact with the oxygen-rich surface water and is gradually used up by decaying organic material, washed into the water when it rains. This results in low dissolved oxygen levels in the lower layers of the water column. Hydro turbine intakes typically draw water from the lower part of the upstream reservoir, so this can also create low-oxygen conditions immediately downstream of hydropower dams, especially during late summer and early fall.

To address this problem, TVA has installed special aeration equipment at many of its dams to add oxygen to the water. This equipment varies depending on conditions at each dam. At some dams, TVA uses pumps resembling big ceiling fans to push warm, oxygen-rich surface water downward, where it is mixed with low-oxygen bottom water and then drawn in by the turbines during generation. At other dams, TVA has installed oxygen-injection systems (a large-scale version of the same basic principle that aerates your home aquarium—only using liquid oxygen instead of air); constructed aerating weirs which mimic a natural waterfall, adding oxygen to the water as it plunges over the top of the weir walls; and installed auto-venting turbines which use low-pressure areas to draw air into the water as power is being generated.

“We keep a close watch on oxygen levels and other water quality conditions throughout the summer,” says TVA River Forecast Center manager Susan Jacks. “That way, we can minimize any problems that may develop by adjusting flows or stepping up the use of aeration equipment. That’s an enormous benefit to aquatic life, as well as to municipal and industrial water users who depend on the river’s assimilative capacity—especially during hot, dry summers such as the one we are currently experiencing.”

The arrival of fall, with shorter days and cooler air temperatures, cools the surface water, gradually allowing it to blend with more and more of the water column. By late October or November, the reservoir has “turned over”—meaning temperatures and DO levels are equalized from surface to bottom.

TVA will begin lowering the level of Blue Ridge Reservoir in Fannin County, Georgia in mid July in order to rehabilitate the dam.

The drawdown will occur gradually over a 16-week period with the goal of lowering the reservoir to an elevation of 1,630 feet above sea level (57 feet lower than the usual summer elevation) by November. The reservoir will be kept at an elevation between 1,620 and 1,630 feet above sea level until planned repairs to the penstock (the underwater pipe that carries water from the reservoir to the turbines in the powerhouse) can be completed.

TVA expects to complete the repairs in time to begin refilling the reservoir in April or mid-May 2011, although unexpected events such as sustained heavy rains could prolong the drawdown.

TVA is installing a new liner in the penstock, which was damaged due to excessive water pressure the first time it was filled with water in 1931. TVA, which acquired the dam from Tennessee Electric Power Company in 1939, has periodically lowered the water elevation for dam safety inspections due to the deteriorating condition of the penstock.

The penstock repairs will begin during the regularly scheduled deep drawdown for the 2010 dam safety inspection and should eliminate the need for regularly scheduled deep drawdowns in the future.

TVA also will reinforce the intake tower and stabilize the upstream and downstream sides of the earthen dam as part of the project.

The links below will take you to reservoir-related information on TVA’s website.

TVA’s reservoir operating policy: Learn how TVA manages the flow of water through the Tennessee River system to provide navigation, flood damage reduction, power supply, water quality, water supply, recreation, and other benefits.

Reservoir information: Get detailed information about individual reservoirs, including observed and predicted elevations and releases at TVA dams, reservoir operating guides, water quality improvements, fish population survey results, and more. To check reservoir information from your cell phone or other mobile device, go to http://m.tva.com.

Rainfall and stream flows: Get the latest information on daily rainfall and stream flows across the Valley.

Map of TVA reservoirs and power plants: Our interactive map is your guide to the entire TVA power system, including fossil and nuclear plants, dams and reservoirs, and visitor centers. You’ll find interesting facts about each facility and learn how they work together for the purposes of power supply, river management, and economic development.

Water supply FAQs: Get answers to frequently asked questions about obtaining a water intake permit, improving water quality around intakes, inter-basin transfers, and more.

Dangerous areas around TVA dams: If you like fishing or enjoy swimming and boating on TVA-managed reservoirs, you need to be aware of the possible hazards around dams, locks, and powerhouses.

How to lock through: Find out what you need to do to safely approach a navigation lock, secure your boat in the lock chamber, and exit the lock.